This invention pertains to thermosetting powder paints and particularly to thermosetting polymeric binders comprising glycidyl functional acrylic polymers adapted to crosslink with silanol or methylol functional solid cyclic silicone resin useful as binder in powder paints.
Acrylic resins useful in powder coatings are known to exhibit good weathering properties if the acrylic copolymer does not contain major amounts of copolymerized styrene. U.S. Pat. No. 4,446,259 and U.S. Pat. No. 4,472,484 disclose blends of acrylic binder and silicone resin. Acrylic copolymers of silicone resin should provide considerably improved weathering properties and should offset other film property deficiencies of silicone resins. However, attempting to coreact acrylic copolymer with silicone resins is difficult to achieve in practice since the reaction is difficult to control and often results in useless crosslinked polymers known as gels. The reaction is difficult to control due to hydroxyl groups on the acrylic polymer as well as the silanol groups on the silicone polymer. For instance, a hydroxylated acrylic copolymer prepared in solvent and subsequently reacted with silanol or alkyl ether groups in silicone resin either gelled upon coreaction or gelled during the process of stripping solvent off the resulting product. Efforts to blend hydroxyl functional silicones with a wide variety of polymers including acrylic polymers resulted in paint films with greatly reduced mechanical properties with little or no improvement in exterior weathering.
It now has been found that a glycidyl functional acrylic polymer blended with silanol or methylol functional solid cyclic silicone provides an excellent thermosetting polymeric binder useful in powder coating to produce cured paint films exhibiting superior exterior weathering properties. Using the solid cyclic silicone as crosslinker avoids the need for undesirable isocyanate or diacid crosslinkers and further overcomes the inherent deficiencies associated with conventional liquid silicones. The polymeric blends of this invention advantageously can be formulated into powder paints to provide a low or variable gloss appearance to durable exterior powder coatings. Substantially improved chalk resistance and exterior weathering properties are obtained from the blended polymers along with other improved film integrity properties. The polymeric blend can be combined with other additives to produce clear or pigmented powder coatings. These and other advantages of this invention will become more apparent by referring to the detailed description of the invention and the illustrative examples.